Keep chat scrolling under user control

src/app.rs

@@ -83,6 +83,8 @@ pub(crate) struct App {
     pub(super) composer: String,
     pub(super) queued_inputs: VecDeque<String>,
     pub(super) conversation: Vec<ChatMessage>,
+    /// Follow appended chat content until the user scrolls away from the tail.
+    pub(super) chat_follow_tail: bool,
     active_turn: Option<generation::TurnSummary>,
     pub(super) a2ui: crate::a2ui::Store,
     pub(super) a2ui_history: Vec<crate::a2ui::Store>,
@@ -310,6 +312,7 @@ pub(crate) enum Message {
     SubmitPrompt,
     StopGeneration,
     GenerationTick,
+    ChatScrolled(scrollable::Viewport),
     ChooseProjectFolder,
     ProjectFolderPicked(Option<PathBuf>),
     ProjectNameChanged(String),
@@ -407,6 +410,7 @@ impl App {
                         composer: String::new(),
                         queued_inputs: VecDeque::new(),
                         conversation: Vec::new(),
+                        chat_follow_tail: true,
                         active_turn: None,
                         a2ui: crate::a2ui::Store::default(),
                         a2ui_history: Vec::new(),
@@ -535,6 +539,7 @@ impl App {
             composer: String::new(),
             queued_inputs: VecDeque::new(),
             conversation: Vec::new(),
+            chat_follow_tail: true,
             active_turn: None,
             a2ui: crate::a2ui::Store::default(),
             a2ui_history: Vec::new(),
@@ -729,6 +734,7 @@ impl App {
             }
             Message::ShowChat => {
                 self.detail_tab = DetailTab::Chat;
+                self.chat_follow_tail = true;
                 return scroll_chat_to_end();
             }
             Message::ShowA2ui => self.detail_tab = DetailTab::A2ui,
@@ -1069,6 +1075,7 @@ impl App {
                             serde_json::to_string(&action).unwrap_or_default(),
                             self.a2ui.client_metadata()
                         );
+                        self.chat_follow_tail = true;
                         self.start_generation();
                         return scroll_chat_to_end();
                     }
@@ -1204,6 +1211,7 @@ impl App {
                 }
             }
             Message::SubmitPrompt => {
+                self.chat_follow_tail = true;
                 self.start_generation();
                 return scroll_chat_to_end();
             }
@@ -1236,13 +1244,19 @@ impl App {
             Message::GenerationTick => {
                 #[cfg(target_os = "macos")]
                 self.poll_titling();
-                let changed = self.poll_generation();
+                self.poll_generation();
                 let images = self.load_next_a2ui_image();
-                if changed {
-                    return Task::batch([scroll_chat_to_end(), images]);
-                }
                 return images;
             }
+            Message::ChatScrolled(viewport) => {
+                let offset = viewport.absolute_offset();
+                let reversed = viewport.absolute_offset_reversed();
+                let follow = chat_near_end(self.chat_follow_tail, offset.y, reversed.y);
+                if follow != self.chat_follow_tail {
+                    self.chat_follow_tail = follow;
+                    return iced::widget::operation::scroll_to(chat_scroll_id(), reversed);
+                }
+            }
             Message::ChooseProjectFolder => {
                 self.choosing_folder = true;
                 return Task::perform(
@@ -1486,6 +1500,7 @@ impl App {
                             return Task::none();
                         }
                         self.conversation = messages.into_iter().map(ChatMessage::from).collect();
+                        self.chat_follow_tail = true;
                         generation::promote_legacy_turn_summaries(&mut self.conversation);
                         self.context_notice = None;
                         self.clear_a2ui();
@@ -2121,7 +2136,19 @@ fn focus_composer() -> Task<Message> {
 }
 
 fn scroll_chat_to_end() -> Task<Message> {
-    iced::widget::operation::snap_to(chat_scroll_id(), scrollable::RelativeOffset::END)
+    iced::widget::operation::scroll_to(
+        chat_scroll_id(),
+        scrollable::AbsoluteOffset::<f32>::default(),
+    )
+}
+
+fn chat_near_end(following: bool, offset: f32, reversed_offset: f32) -> bool {
+    const TAIL_THRESHOLD: f32 = 24.0;
+    if following {
+        offset <= TAIL_THRESHOLD
+    } else {
+        reversed_offset <= TAIL_THRESHOLD
+    }
 }
 
 /// Deletes session checkpoints whose session is gone. Deleting a session or a
@@ -2255,6 +2282,14 @@ mod tests {
         assert_eq!(section_offset(field(990.0), viewport, 500.0), 390.0);
     }
 
+    #[test]
+    fn chat_only_follows_updates_while_near_the_tail() {
+        assert!(chat_near_end(true, 0.0, 500.0));
+        assert!(!chat_near_end(true, 25.0, 475.0));
+        assert!(chat_near_end(false, 500.0, 24.0));
+        assert!(!chat_near_end(false, 400.0, 100.0));
+    }
+
     #[test]
     fn ds4_gib_and_streaming_cache_inputs_are_typed() {
         assert_eq!(